A precise balance between the deposition and degradation of extracellular matrix (ECM) molecules, including collagen I and fibronectin, is required for normal tissue function, and is a key component of normal tissue repair. Excessive or inappropriate deposition of ECM molecules disrupts normal tissue architecture, leading to altered tissue mechanics and impaired organ function. The mechanisms that control ECM deposition and turnover are incompletely understood. Our data demonstrates that fibronectin matrix polymerization regulates the deposition and retention of several ECM molecules, including collagen I. Furthermore, fibronectin matrix polymerization regulates the composition and stability of cell-ECM fibrillar adhesions, enhances cell contractility, and increases the mechanical strength of a collagen-based tissue construct. Our studies also indicate that the structural organization of ECM fibronectin and collagen I depends upon the continuous polymerization of a fibronectin matrix. Agents that disrupt fibronectin polymerization trigger enhanced fibronectin and collagen I turnover; these agents also induce turnover of fibronectin in tissues. This data suggests that ECM turnover is regulated, in part, by fibronectin polymerization itself. Our preliminary data indicate that fibronectin matrix turnover involves caveolin-1 mediated endocytosis and lysosomal degradation. In this proposal, we will investigate the mechanisms by which fibronectin matrix accumulation is controlled, and determine the functional consequences of fibronectin matrix remodeling. We will use our in vitro model system employing fibronectin-null myofibroblasts in conjunction with recombinant mutant fibronectins and fibronectin fragments to determine the mechanisms by which fibronectin matrix polymerization controls the turnover and endocytosis of fibronectin. We will also test how fibronectin matrix polymerization regulates the formation and stability of cell-matrix fibrillar adhesion sites in cultured cells and in tissues. We have also established a tensile testing system to quantitatively determine the mechanical properties of tissues and collagen-based tissue constructs. We will use this system to determine the effects of fibronectin matrix turnover on cell contractility, cell tension generation, and the mechanical strength of tissues. Determining the mechanisms by which fibronectin polymerization regulates ECM remodeling and tissue mechanical strength will provide important insights into factors that contribute to the development of fibrosis, and into mechanisms that could lead to restoration of normal matrix architecture and improved organ function in individuals with fibrotic disorders.